1. Field of the Invention
This invention pertains to the optical fiber terminations whereby damage to the fiber, and particularly to the microstructured region thereof containing channels separated by thin walls, is eliminated and/or reduced.
2. Description of Related Art
In developing the hollow core photonic band gap fibers for various applications, the most important of which is for transmission of ultra-high laser power in the infrared, on the order of tens of giga watts per square centimeter, the fibers are intended for implementation in missile warning protection systems for military and commercial aircraft. The fibers can also be used in laser surgery with optical beams at various wavelengths, including wavelengths of 2.94 μm and 10.6 μm. These fibers typically consist of a hollow, air-filled core and a microstructured cladding surrounded by a solid glass jacket for mechanical strength as shown in FIG. 1. The microstructured cladding typically consists of multiple channels or veins around the core which are separated from each other by webbing or walls, the thickness of which is on the order of a few hundred nanometers. Air fill fraction of the microstructured region is up to about 90%, i.e., air to glass ratio. Although over 99% of the light is predicted to propagate in the fiber core, where a laser beam can be focused into, there is always a risk of laser misalignment or beam shift due to temperature or environmental changes. If the beam impinges on the microstructured region, damage or complete destruction usually occurs whereby the fiber is ablated or otherwise damaged and cannot be used for optical transmission.
Hollow core silica fibers with a structured region providing a photonic band gap have been known since about 1999. Photonic band gap fibers have recently been demonstrated using silica glass with a minimum loss of about 1-7 dB/km at 1.5 μm. It would be obvious to use hollow core photonic band gap silica fiber to transmit wavelengths longer than 1.5 μm since it was believed that the light was guided in the hollow core. However, it turns out that hollow core photonic bandgap silica fibers do not transmit well beyond 2 μm since up to several percent of the light can propagate in the microstructured region and this light is highly attenuated through multiphonon absorption in silica. Hence, it is not possible to use silica photonic bandgap fibers for high power transmission in the infrared region beyond a wavelength of about 2 μm.